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302________ Chapter 15 ' __________ 1945'Guide i' [ , Since the system is to deliver 60 Mbh with a temperature difference of 20 F, the pump, must circulate 60 10, or 6 gpm. For this load, the unit/A for a l } in. pipe is 86 milinches. The main circuit consists of 110 ft of.pipe and 10 elbow equivalents and may be placed equal to 136 ft of 1J4 in. pipe. The total Jh for the main circuit is 136 x 86, or 11,696 milinches, or practically 1 ft for a flow of 6 gpm. ''" At1 the points A, a portion of the water will be diverted through the radiator circuit, and as a result less than 6 gpm will flow in the main between the points A and B, and the fh will be slightly less than 86 milinches per foot between these points. But the difference will be so small that it may be neglected and the total/ft from A to B assumed to be .4 x 86, or 344 milinches; The ph forcing the water through the radiator circuit will then be 344 milinches. The radiator circuit consists of 11 ft of pipe and about 14 elbow equivalents and may be placed equal to 32 ft of pipe and the available ph 344 H- 32, or about 11 milinches per foot. With this ph, a in. pipe will convey about 5 Mbh (Fig. 3),or 0.5 gpm. Hence, only 5 -5- 60, or about 8 per. cent of the water, would flow through the radiator, if the radiator's gravity head is not considered. The water would, then, have to cool 60 F in order to deliver 15 Mbh, and the average radiator temperature Fig. 13. One-pipe Forced Circulation System would be 170 if the water entered at 200. This would require a large radiator and result in an unsatisfactory installation. To secure a larger flow of water through the radiator it is necessary to increase the /A of the short path A-B in the main. This may be done by inserting special resistance tees at points A and B, or by inserting an orifice resistor between points A and B, or by reducing the 134 in. main between the points A and B to the next smaller size, 1 in. The'relative quantity of water flowing through the radiator may then be found by trial calculations: Assume, first, that 1 gpm will flow through the radiator and 5 gpm through the main. The ph for 1 gpm and a in. pipe is 40 milinches per foot, or 32 x 40, or 1280 for the radiator circuit. The main circuit consists of 4 ft of 1 in. pipe and two reducing tees. The two reducing tees may be placed equal to 0.8 elbow equivalents (Table 3), and the equivalent length of the main circuit equal to 5.7 ft. The/ft for 5 gpm and a 1 in. pipe is 240 milinches per foot, or 5.7 x 240, or 1370 milinches.for the main, circuit. Since this is only slightly more than the calculated /ft for the radiator circuit, it is evident that the flow through the radiator will be slightly more than 1 gpm, and it is not necessary to make a second trial calculation. The quantity of water flowing through the radiator can be varied by varying the distance between the points A and B, where the radiator branches join the main. In order to deliver 15 Mbh to the radiator with a temperature difference of 20, it is necessary that 1.5 gpm flow through the radiator; since, m this case, the flow through the radiator is only 1 gpm, the temperature difference must be 30 F. If the water enters the radiator at 190. F, the average water temperature will be 175 F. The quantity of. water circulating through the radiator may be varied con siderably without an appreciable effect on the quantity of heat dissipated by the radiator. This is. evident from the following calculation. Assume that Radiator 2 has been designed so that it will dissipate 15 Mbh when its flow of water is at the rate of 1 gpm, and when its average temperature is 175 F. Assume Hot Water Heating Systems, and Pipin'/' 303 ---------------------------------------------------------------------- 1000'-----------------------------------------:------------ ------------:---------- * 5"_______ 5** 5_______ . 17 16 3150 15 2700 14 2250 13 d 4" a * a a f^tfr55 tn 900 m1350 da tn ^ cn tn7 i h Pipe sizes . s* 5" 1 !~ *o !> Mbh for each pipe 1800-* 0 CD 3150 CD 2700 CD 2250 CD 1 4" U' ,1" 6"l \ 9 450 10 900 11 1350 12 7 * or,' 8 3600 Mbh n2I r'* 8" pipe . Fig. 14. Two-Pipe Reversed Return System that the flow of water is increased 50 per cent---from 1 gpm to 1.5 gpm. The water will then flow through the radiator in two-thirds the time and will cool two-thirds as much; i.e., it will cool 20 F instead of 30 F, and the average radiator temperature will be 180 F , instead of 175 F. If the surrounding temperature is 70 F, the temperature differences, radiator and surroundings, will be 110 and 115 F, respectively. Consequently, the heat dissipation will be increased only about 6 per cent when the quantity of. water flowing through the radiator is increased 50 per cent. By decreasing the main from 1 yf tp 1 in. between radiator branches while the flow is . decreased from 6 to 5 gpm, the fh in that section of the main is increased from 344 to 1370 milinches, or 926 milinches. Hence, for the four radiator sections the increase is 3704 milinches, and the total/ft for the circuit will be 11,696 plus 3704, or 15.4 in. instead of 11.7 in. as first calculated. The pump must, therefore, circulate 6 gpm against a head of 1.3 ft. Reversed and Direct Return Systems In a reversed return system the radiators are connected so that all circuits are practically of equal length and so that the water flowing through the radiator nearest the boiler must travel practically as far as the water flowing through the radiator farthest from this boiler, as illus- ; trated in Fig. 14. In a direct return system the radiators are connected so that all water returns to the boiler along the most direct path after it has passed through its radiator, as illustrated in Fig. 15. Example 7. In Fig. 14, sixteen air conditioning units, each demanding 450 Mbh, are to be supplied with water from a central plant. The system is divided into two equal parts as shown. Each part supplies eight units and has, therefore, eight circuits. The total length of each of the eight circuits is about 1170 ft. Solution. If the total/A is to be about 15 ft, the unit/ft must be about 150 milinches per foot. With this preliminary estimate, pipe sizes may be selected from Fig. 3 and recorded with corresponding calculations as shown in Table 7 for Circuit 8, from which it appears that the fh of this circuit is 188,570 milinches, or 15.7 ft. . In order that each of the eight air conditioning units may receive an equal supply of J' ater, the /ft of each of the remaining circuits must also be 15.7 ft. Since all pipe sizes have been selected as shown in Example 6, any adjustments that may be necessary must be made in the connections from the main through the air conditioning unit and back to the main. For Circuit 8 the friction head through the air conditioning unit was assumed to be 20,000 milinches. For Circuit 4, for example, a tabular calculation like . d4" *1 pV 5" -3f------------- H" 450 Mbh 10 900 11 1350 12 CD CD . Pipe sizes 2F 9j--j 450Mbh[t] 900 Mbh for each pipe 1350 [t] i 1 , . 1800- jg P * 5" "** S 6" 0 \ CD 3150 CD 2700 CD 2250 CD 1 5" V' If7 ^ 16 3150 Is 2700 "Sf- 2250 13*` Fig. 15. 8 |8 3600 Mbh |r* 8" pipe Two-Pipe Direct Return System